Mate I don't have a problem with a transition at 18000. It operates in quite a few other places in the world quite fine with me.
In fact it will save all the ball ache of SID's with potential for alt busts due to changing to FL level offs.
The issue really isn't with VFR its the amount of IFR which bangs around lower than 18000ft in the UK. Alot of approaches cross departures from other airports and even FIR's.
In the UK we have CAS class A airways down to FL80.